Method for producing a high-strength, high-toughness silicon nitride sinter

ABSTRACT

A method for the production of a high-strength high-toughness silicon nitride sinter includes the steps of mixing a silicon nitride powder with a sintering additive, adding to the resultant mixture as seed particles 0.1 to 10% by volume, based on the amount of the mixture, of elongated single crystal β-silicon nitride particles having a larger minor diameter than the average particle diameter of the silicon nitride powder and having an aspect ratio of at least 2, forming the resultant mixture so as to orient the elongated single crystal β-silicon nitride particles as seed particles in a specific direction, and heating the green body to density it and simultaneously induce epitaxial growth of single crystal β-silicon nitride particles, and a high-strength, high-toughness silicon nitride sinter obtained by the method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a silicon nitride sinter exhibiting extremelyhigh strength and toughness in a specific direction and a method for theproduction thereof.

Silicon nitride exhibits higher covalent bond strength and much betterstability at high temperatures than oxide ceramics. This has stimulatedresearch into high-temperature structural materials using siliconnitride. While some practical applications have recently been found forsilicon nitride such as in engine parts, including automobile gradeturbo chargers, use has been limited because, despite being the toughestamong ceramics, silicon nitride has a fracture toughness that is one ormore decimal places lower than that of metallic materials.

2. Description of the Prior Art

Various methods aimed at further improving the toughness of siliconnitride ceramics have been studied. Among these, the method of improvingtoughness by dispersing plate- or column-shaped second phase in theceramic matrix and consequently producing a bridging or an pull-outeffect along cracks occurring in the ceramic matrix proves particularlyeffective. Various procedures have been developed for implementing thismethod. These include the method of dispersing whiskers or platelikeparticles by mechanical agitation and the method of developing coarsecolumnar grains of β-silicon nitride in the sinter as by gas pressuresintering.

The former method produces a highly toughened silicon nitride sinterhaving fracture toughness in the range of from 10 to 14 MPa·m^(1/2). Am.Ceram. Soc. Bull., 65 2! 351-356 (1986), for example, reports formationof a sinter having fracture toughness in the range of from 10 to 12MPa·m^(1/2) by dispersing 10 to 40% in volume of SiC whiskers in asilicon nitride and subjecting the resultant green body to hot presssintering, and Ceramic Transactions, Vol. 19, pp. 765-771 reportsproduction of a sinter having fracture toughness of 14 MPa·m^(1/2) bydispersing 30% in volume of SiC platelike particles in a silicon nitrideand subjecting the resultant green body to a treatment with a hot press.Though the sinters obtained by these procedures have high levels offracture toughness, they exhibit conspicuously low strength (in therange of from 400 to 600 MPa) because the incorporated reinforcingmaterials act as flaws. Besides, the dispersion of 10 to 40% in volumeof a second phase is expensive because it requires firing by a specialmethod such as hot pressing or hot isostatic pressing (HIP).

The latter method consists in firing in an ambience of nitrogencompressed to about 100 atmospheres at a temperature in the range offrom 1800° to 2000° C. thereby developing large elongated β-siliconnitride grains in the sinter that produce the same effect as whiskers.This method eliminates the need for hot press or HIP and forms a siliconnitride sinter having high fracture toughness in the range of from 8 to11 MPa·m^(1/2). This is comparable to the toughness obtained byincorporation of whiskers.

Am. Ceram. Soc. Bull., 65 9! 1311-1315 (1986) reports production of asilicon nitride sinter having fracture toughness of about 9 MPa·m^(1/2)by adding alumina-rare earth oxide as a sintering additives to rawmaterial α--Si₃ N₄ and firing the resultant mixture in an ambience ofnitrogen compressed to 20 to 40 atmospheres at 2000° C., and J. Am.Ceram. Soc., 76 7! 1892-1894 (1993) reports production of a siliconnitride sinter having fracture toughness in the range of from 8.5 to10.3 MPa·m^(1/2) by adding Y₂ O₃ --Nd₂ O₃ as a sintering additives toraw material of β-Si₃ N₄ and firing the resultant mixture in an ambienceof nitrogen compressed to 100 atmospheres at 2000° C. for 2 to 8 hours.The high-toughness silicon nitride sinters obtained by these gaspressure sintering methods have low strength (in the range of from 400to 700 MPa) because large elongated β-silicon nitride grains developingin the sinter act as flaws.

No silicon nitride sinter which combines high strength with hightoughness has yet been developed. It is, therefore, a primary object ofthis invention to provide a silicon nitride sinter combining highstrength with high toughness and enabling production thereof by aninexpensive process, and a method for the production thereof.

SUMMARY OF THE INVENTION

Specifically, this invention relates to a method for the production of ahigh-strength, high-toughness silicon nitride sinter comprising thesteps of mixing a silicon nitride powder with a sintering additive,adding to the resultant mixture as seed particles 0.1 to 10% by volume,based on the amount of the mixture, of rod-like single crystal β-siliconnitride particles having a larger diameter than the average particlediameter of the silicon nitride powder and having an aspect ratio of atleast 2, forming the resultant mixture so as to orient the rod-likesingle crystal β-silicon nitride particles, added as seed particles in aspecific direction, and the green body in heated to density it andinduce epitaxial growth from single crystal β-silicon nitride particles;resulting in the high-strength, high-toughness silicon nitride sinterobtained by this method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a model diagram showing the microstructure of a siliconnitride sinter obtained by this invention.

FIG. 2 is an electron micrograph at 2000 magnifications of a fracturesurface of the sinter of this invention.

FIG. 3 is an electron micrograph at 2000 magnifications of a ground andetched surface of the sinter of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the research that led to this invention will be summarized.

Prior to achieving this invention, the inventors extensively studied theconventional high-toughness silicon nitride sinter obtained by gaspressure sintering for determining the relation between themicrostructure, strength, and fracture toughness of the sinter. Theyfound that the conventional gas pressure sintering causes random growthof elongate grains from β-silicon nitride particles present in the rawmaterial silicon nitride as nuclei and consequently induces occurrenceof large elongate grains in the produced sinter. Since the largeelongate grains are origin for fracture, the sinter presents lowstrength, although it has improved fracture toughness.

In cooperation with their colleagues, the inventors prepared seedcrystals and produced a sinter by repeating the method disclosed in J.Am. Ceram. Soc., 77 7! 1857-1862 (1994). To be specific, they addedrod-like single crystal β-silicon nitride particles morphologicalregulated as seed crystals to silicon nitride raw material and tried tocontrol the shape and size of anisotropically grown elongate β-siliconnitride grains. They consequently succeeded in producing a siliconnitride sinter having relatively high strength in the range of from 900to 1000 MPa and relatively high toughness in the range of from 8.2 to8.6 MPa·m^(1/2). When the seed crystals were enlarged or the amount ofseed crystals added was increased for the purpose of further enhancingthe toughness, however, the strength declined to about 700 MPa in spiteof improvement of the fracture toughness to a level in the range of from9 to 10 Mpa·m^(1/2).

The inventors next investigated the microstructure of the sinter withrespect to the relation between the strength and toughness thereof andconfirmed the following fact.

They made a thorough observation of the microstructure of the systemunder discussion to find that, in a system retaining high strength andacquiring enhanced toughness, a group of large elongated grainsdeveloped from seed crystals were dispersed in a matrix of small grainsand, in a system suffering a decline in strength, the group of largeelongated grains grown from the seed crystals coalesced. Thiscoalescence was suspected of causing an increase in the flaw size.

Based on this knowledge the inventors continued their study on controlof the microstructure by the incorporation of seed crystals. Theyconsequently learned that when the group of large elongated grainspresent in random three-dimensional orientation are aligned in aspecific direction, the toughness of the system is effectively improvedin that direction without decrease in strength. Ultimately, theyobtained the high-strength, high-toughness silicon nitride sinter ofthis invention.

This invention will now be described in detail.

To manufacture the high-strength, high-toughness silicon nitride sinter,this invention requires the raw material powder of silicon nitride to beadded with a prescribed amount of a sintering additive. The siliconnitride raw material may be in any of such crystal systems as α type, βtype, or amorphous type. It is advantageously used in the form of a finepowder having an average particle diameter of not more than 0.5 μm. Thesintering additive may be any of the known compounds available for thepurpose of accelerating the sintering. Concrete examples are MgO, CaO,Al₂ O₃, Y₂ O₃, Yb₂ O₃, HfO₂, Sc₂ O₃, CeO₂, ZrO2, SiO₂, Cr₂ O₃, and AlN.

The combination of these sintering additives and the amount of sinteringadditive to be added vary with the method of firing, which may, forexample, be normal pressure sintering, gas pressure sintering, hotpress, or hot isostatic pressing (HIP). They are so selected that thesample, on being fired by a given method, is compacted to a relativedensity of not less than 97%. For enabling the silicon nitride to attainanisotropic growth in an elongated shape during sintering, the sinteringadditive should appropriately contain a rare earth oxide such as Y₂ O₃or Yb₂ O₃.

The mixing of the raw materials can be conducted using any of thecommercially available equipments available for mixing or blendingpowders. The raw materials are advantageously mixed wet by the use of asuitable solvent such as water, methanol, ethanol, or toluene. In thewet mixing, it is best to use an organic solvent for preventing theotherwise possible oxidation of silicon nitride. In the presence of suchan organic solvent, the mixing can be effectively accelerated by using adispersant such as sorbitan monooleate.

Then, to the slurry obtained as described above, rod-like single crystalβ-silicon nitride particles are added as seed crystals in an amount inthe range of from 0.1 to 10% by volume, preferably from 1 to 5% byvolume. If the amount of the seed crystals so added is less than 0.1% byvolume, the group of elongated grains will not be incorporated in afully satisfactory amount into the sinter. Conversely, if the amountexceeds 10% by volume, the excess of added seed crystals impede thesintering to the extent of preventing the formation of a compact sinterand, though a compact sinter may be attained by pressure sintering suchas hot press, the excessive seed crystals will prevent the producedsinter from acquiring high strength because the group of elongatedgrains grown from the seed particles coalesce and increase the size offlaw. Hence, the amount of the seed crystals added should be limited tothe range of from 0.1 to 10% by volume. The shape of the seed crystalsis preferably such that the diameter is larger than the average particlediameter of the raw material powder of silicon nitride and the aspectratio is not less than 2. If the diameter of the seed crystals issmaller than the average particle diameter of the raw material powder,the seed crystals will be dissolved in the transient liquid duringsintering and will not accomplish their role of seed crystals. If theaspect ratio is not more than 2, the seed crystals will not bethoroughly oriented as in the case of sheet molding and will inducecoalescence between the randomly grown elongated grains and consequentlycause the produced sinter to suffer a decrease in strength. The upperlimit of the aspect ratio is about 50. If the aspect ratio exceeds 50,the seed crystals will not be thoroughly dispersed.

The elongated single crystal β-silicon nitride particles used as seedcrystals may be commercially available β-silicon nitride whiskers.However, since these whiskers lack uniformity of size and containlattice defects and impurities, it is better to use rod-like singlecrystal β-silicon nitride particles of high purity and uniform sizemanufactured by a method such as that reported in Journal of CeramicSociety of Japan, 101 9! 1071-1073 (1993). It is important that theaddition of the seed crystals to the raw material powder be implementedby mixing the silicon nitride raw material thoroughly with the sinteringadditive in accordance with the wet mixing technique mentioned above andcausing the seed crystals to be dispersed in the resultant slurry bymeans of ultrasonic dispersion or by the pot mixing technique using aresin pot and coated resin balls in such a manner as to avoid breakingthe seed crystals.

Then, the mixed slurry obtained as described above and a proper amountof an organic binder added thereto are mixed. The produced mixture issheet molded by the use of a doctor blade or formed by the use of anextrusion device to effect orientation of the seed crystals in themixture. Particularly when the mixture is sheet molded, the producedsheet are stacked using a hot plate press to acquire a prescribedthickness.

Subsequently, the formed mixture is calcined by the ordinary firingschedule, i.e. at a temperature in the approximate range of from 600° to1000° C. to remove the binder and then fired in the ambience of nitrogenkept at a temperature in the range of from 1700° to 2000° C. under apressure of 1 to 200 atmospheres. For the purpose of obtaining a sinterto manifest high strength and high toughness, it is important to sinterto a relative density of not less than 97% and that the elongatedβ-silicon nitride grains be thoroughly developed from the seed crystals.The silicon nitride sinter obtained from the raw material whichincorporates the seed crystals possesses a microstructure in which largeelongated β-silicon nitride grains epitaxially grown from the seedcrystals are two-dimensionally dispersed in a matrix of small siliconnitride grains. It is important that the group of these large elongatedgrains account for a volume ratio of not less than 10%. If the volumeratio of the group of elongated grains after the firing is less than10%, the level of improvement of toughness will be unduly low and thedesired sinter will not be obtained. The specific temperature, nitrogenpressure, and keeping time during the firing are closely related to thesintering additive. It is, therefore, necessary to decide the optimumconditions for enabling a given sintering additive to fulfill therequirements mentioned above and for enabling the produced sinter tomanifest high strength and high toughness by preliminary testing, forexample. This high-toughness silicon nitride sinter is characterized bybeing compacted by pressureless firing or gas pressure firing.Optionally, the densification may be effected by hot press or HIP.

The silicon nitride sinter produced by the method of this invention asdescribed above possesses such a microstructure that elongated β-siliconnitride grains grown epitaxially from β-silicon nitride particles asseeds are highly dispersed in a planar distribution. Owing to themicrostructure of the sinter which has the elongated grains oriented ina planar distribution as described above, this sinter acquires enhancedstrength in a direction perpendicular to the direction of theorientation (1) because the whole group of elongated grains functiontoward enhancing the toughness and consequently the level of improvementof toughness is high as compared with the conventional high-toughnesssilicon nitride in which the group of elongated grains are present inrandom three-dimensional orientation and (2) because the large elongatedgrains, though present in the texture of the sinter, are dispersed in aplanar distribution and therefore the extent to which they act as flawis small.

This invention allows production of a silicon nitride sinter whichacquires compaction to a relative density of not less than 99% andexhibiting a strength of not less than 1100 MPa and a fracture toughnessof not less than 11 MPa·m^(1/2) in a direction perpendicular to thedirection of orientation of the elongated grains.

The microstructure of the silicon nitride sinter obtained by thisinvention is shown by a model diagram in FIG. 1.

In FIG. 1, the symbol "a" represents a small silicon nitride grains, thesymbol "b" an rod-like single crystal β-Ni₃ N₄ particles as a seedcrystal, and the symbol "c" elongated grains grown epitaxially from aseed crystal.

The method disclosed by this invention enables production of a siliconnitride sinter exhibiting strength of not less than 1100 MPa andfracture toughness of not less than 11 MPa·m^(1/2) in a directionperpendicular to the direction of orientation of the elongated grainsand consequently permits provision of a silicon nitride ceramicsimultaneously presenting strength and toughness of a high levelunattainable by conventional silicon nitride ceramics.

The silicon nitride sinter of this invention therefore possessesoutstanding reliability as compared with conventional silicon nitridesinters and can be expected to find extensive utility as a structuralmaterial for heat exchangers, engines, and gas turbine parts in theplace of refractory alloys.

The invention will now be described below with reference to workingexamples and comparative examples.

Example (Production of seed Crystals)

In a planetary mill using balls and a pot both made of silicon nitride,30 g of a raw material powder of α-Si₃ N₄ having a specific surface areaof 2 m² /g and 2.418 g of Y₂ O₃ and 1.288 g of SiO₂ added thereto weremixed in methanol as a mixing medium (Composition A). Similarly, 30 g ofa raw material of α-Si₃ N₄ having a specific surface area of 5 m² /g and2.418 g of Y₂ O₃ and 0.322 g of SiO₂ added thereto were mixed(Composition B). The compositions A and B were each treated with avacuum evaporator to vaporize methanol, further vacuum dried at 120° C.,and passed through a 60-mesh sieve to obtain a composite for thepreparation of seed crystals. The composite was placed in a cruciblemade of silicon nitride and heated therein in an ambience of nitrogenunder 5 atmospheres at 1850° C. for two hours. The aggregateconsequently obtained was crushed into a powder of 60 mesh.

The powder obtained as described above was sequentially treated with anaqueous solution of hydrofluoric acid-nitric acid mixture (hydrofluoricacid:nitric acid:water=45:5:50 in volume percentage), sulfuric acid,dilute hydrofluoric acid, and aqua ammonia in the order mentioned toremove Y₂ O₃ and SiO₂, glass phase components, and obtain rod-likesingle crystal β-silicon nitride particles. From Composition A, rod-likesingle crystal β-silicon nitride particles having a diameter of 1.4 μmand an aspect ratio of 4 (Seed Crystals SA) were obtained. FromComposition B, rod-like single crystal β-silicon nitride particleshaving a diameter of 0.9 μm and an aspect ratio of 10 (Seed Crystals SA)were obtained. These two lots of seed crystals both possessed extremelyhigh purity as evinced by the fact that the oxygen content was not morethan 0.26% and the yttrium content was not more than 1.3 ppm.

EXAMPLES 1-5 (Production of sinter of this invention)

In a planetary mill using balls and a pot both made of silicon nitride,a raw material powder of α-Si₃ N₄ (having a specific surface area of 10m² /g and an average particle diameter of 0.1 μm) and a sinteringadditive composed of Y₂ O₃ and Al₂ O₃ and 3% by weight of a dispersantproduced by Kao Co., Ltd. and marketed as "Diamine RRT"! based on thetotal of the other three components added thereto were mixed for threehours in a toluene-butanol liquid mixture (80% by volume of toluene and20% by volume of butanol) as a mixing medium.

The amount of the mixing medium per 100 g of solids was 110 cc. Theamounts (% by weight) of the seed crystals Y₂ O₃ and Al₂ O₃ based on thetotal amount of solids (α-Si₃ N₄, seed crystals Y₂ O₃ and Al₂ O₃ ) inthe working examples were as shown in Table 1.

The slurry obtained in each example and the relevant seed crystals addedin an amount of 2 or 5% by weight based on the total amount of solidswere mixed for 24 hours by the use of a resin pot and resin-coatedballs. Further, the resultant mixture and 9% by weight of a binder(polyvinyl butyral resin) and 2.25% by weight of a plasticizer (dioctyladipate) based on the total amount of solids which were added theretowere mixed for 48 hours. The slurry consequently obtained was formed bythe doctor blade method into a green sheet having a thickness of 150 μm.

When this green sheet was observed under an electron microscope, theseed crystals SA and SB were both found to be oriented in a planar formin the plane of the sheet. The green sheet was cut into rectangles of45×50 mm and 50 such rectangles were superposed in one direction andlaminated at 130° C. under a pressure of 70 kg/cm². The resultantlaminate was calcined in a stream of a mixed gas of 95% N₂ and 5% H₂ at600° C. for two hours to remove the organic binder. The calcined sheetwas placed in a carbon crucible, covered with a Si₃ N₄ powder, andretained in an ambience of nitrogen compressed to 9 atmospheres at 1850°C. for six hours, to obtain a sinter of this invention.

The sinter thus obtained was cut into two types test pieces measuring3×4×40 mm. One type was cut so that the sheet forming directioncoincided with the longitudinal direction of the test pieces (Adirection) and the other type was cut so that the directionperpendicular to the sheet forming direction coincided with thelongitudinal direction of test pieces (B direction). After polishing,the test pieces were tested for specific gravity, for room temperaturefour-point bending strength as specified by JIS (Japanese IndustrialStandards) R-1601, and for fracture toughness by the SEPB methodspecified by JIS R-1607. A sample was mirror ground and then etched byimmersion in an equimolar mixed solution of NaOH and KOH at 280° C. for15 minutes. In the etched surface of the sample, the ratio of surfacearea of the group of large elongated grains grown from seed crystals wasmeasured. The results of these tests are shown in Table 1. The electronmicrograph (2000 magnifications) of a fractures surface of the sintermentioned above is shown in FIG. 2 and the electron micrograph (2000magnifications) of a ground and etched plane of the same sinter is shownin FIG. 3. In the photographs, the symbol "d" represents the directionof sheet lamination and the symbol "e" the direction of sheet formation.The density in Table 1 represents the relative density (%) based on therelevant theoretical density. In FIG. 2, the symbol "A" represents alarge elongated grain (grown from a seed crystal) and the symbol "B" amatrix of small silicon nitride grains. In FIG. 3, the symbol "A'"represents an elongated grain and the symbol "B'" a matrix of smallsilicon nitride grains. The photographs show that the elongated grainshad a prefered orientation.

The doctor blade method, JIS R-1601, and JIS R-1607 mentioned above willnow be explained.

Doctor blade method: A slurry is prepared by dissolving an organicbinder in a solvent and dispersing a given ceramic raw material in theresultant solution. The slurry is spread thin on a carrier film by theuse of a blade. The spread layer of the slurry is dried to remove thesolvent and obtain a molded sheet having the ceramic raw material powderfixed by the organic binder.

JIS R-1601 (Four-point bending strength measurement): A test piece isplaced on two supporting points (lower supporting points) separated by aprescribed distance and a load is applied as split between two points(upper load points) of the test piece separated by equal distances inopposite directions from the center thereof between the supportingpoints to find the maximum bending stress at the time the test piecefractures. According to JIS R-1601, the distance between the lowersupporting points (outer span) is 30 mm, the distance between the upperloading point (inner span) is 10 mm, and the total length, width, andthickness of the test pieces are respectively not less than 36 mm,4.0±0.1 mm, and 3.0 ±0.1 mm.

JIS R-1607 (SEPB method, single-edge-precracked-beam method): Thefracture load of a test piece is determined by precracking the testpiece and subjecting this test piece to a three-point bending fracturetest. The magnitude of fracture toughness of the test piece isdetermined based on the precrack length, the size of the test piece, andthe distance between the bending supporting points. According to JISR-1607, the distance between the supporting points is 16 or 30 mm, thewidth of the test piece is 4.0±0.1 mm, the thickness of the test pieceis 3.0±0.1 mm, and the precrack length is 1.2 to 2.4 mm in thethree-point fracture test.

COMPARATIVE EXAMPLE 1

In a planetary mill using balls and a pot both made of silicon nitride,a raw material powder of α-Si₃ N₄ and a sintering additive composed ofY₂ O₃ and Al₂ O₃ and 0.5% by weight of a dispersant (produced by LionChemicals Ltd., Japan and marketed as "Reogard GP") based on the totalof the other three components added thereto were mixed in methanol as amixing medium for three hours. The raw material used was the same as inExample. The composition of the components used was as shown in Table 1.The resultant mixture was dried with a vacuum evaporator to vaporizemethanol. The dry residue was calcined in a stream of a mixed gas of 95%N₂ and 5% H₂ at 600° C. for two hours to remove the organic components.The resultant mixed powder was preformed in the shape of a rectangularcube 42×47×5 mm by the use of a metal mold and further subjected to CIPforming (cold isostatic pressing) under a pressure of 500 MPa. Theshaped solid thus manufactured was fired under the same conditions as inExample. The produced sinter was rated in the same manner as in Example.The results are shown in Table 1.

COMPARATIVE EXAMPLES 2-5

In a planetary mill using balls and a pot both made of silicon nitride,a raw material powder of α-Si₃ N₄ and a sintering additive composed ofY₂ O₃ and Al₂ O₃ and 0.5% by weight of a dispersant (produced by LionChemicals Ltd., Japan and marketed as "Reogard GP") based on the totalof the other three components added thereto were mixed in methanol as amixing medium for three hours. The resultant slurry and a prescribedamount of seed crystals added thereto were mixed for 24 hours by the useof a resin pot and resin-coated balls. The raw materials used hereinwere the same as in Example and the composition of the components was asshown in Table 1. The resultant mixture was dried to vaporize methanol,treated for removal of the organic components, and then molded and firedby following the procedure of Comparative Example 1. The sinterconsequently obtained was rated in the same manner as in Example. Theresults are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                          Properties of sinter                                                Seed crystals                                                                           Den-   Fracture                                                                            Elongated grains                                          Amount added                                                                         sity                                                                             Strength                                                                          toughness                                                                           Ratio                                                                               Property of                 Molding method                                                                        Sintering additive                                                                         Kind                                                                             (% by weight)                                                                        (%)                                                                              (MPa)                                                                             (MPa · m.sup.1/2)                                                          surface                                                                             orientation          __________________________________________________________________________    Comparative                                                                          Metal mold Press                                                                      5 wt % Y.sub.2 O.sub.3, 2 wt % Al.sub.2 O.sub.3                                            None      99.3                                                                             1000                                                                              6.6   None                       Experiment 1                                                                  Comparative                                                                          "       "            SA 2      99.2                                                                             890 8.7   31    None                 Experiment 2                                                                  Comparative                                                                          "       "            "  5      98.3                                                                             780 8.8   39    None                 Experiment 3                                                                  Comparative                                                                          "       "            SB 2      99.0                                                                             890 7.9   21    None                 Experiment 4                                                                  Comparative                                                                          "       "            "  5      97.5                                                                             760 8.7   30    None                 Experiment 5                                                                  Example 1                                                                            Sheet Sekisou                                                                         5 wt % Y.sub.2 O.sub.3, 2 wt % Al.sub.2 O.sub.3                                            SA 2      99.3                                                                             1150                                                                              11.5  32    Exist                                                         *1151                                                                             *11.0                            Example 2                                                                            "       "            "  5      99.1                                                                             1130                                                                              12.0  42    Exist                Example 3                                                                            "       "            SB 2      99.3                                                                             1200                                                                              11.0  23    Exist                                                         *1150                                                                             *11.0                            Example 4                                                                            "       "            "  5      99.2                                                                             1140                                                                              11.5  33    Exist                Example 5                                                                            "       6 wt % Y.sub.2 O.sub.3, 2 wt % Al.sub.2 O.sub.3                                            "  "      99.3                                                                             1150                                                                              11.7  32    Exist                __________________________________________________________________________     Note:                                                                         In the columns titled "strength" and "toughness", the unmarked numerical      values represent data obtained of samples cut in the A direction and the      asterisked (*) numerical values represent data obtained of samples cut in     the B direction.                                                         

Table 1 shows that the sinters obtained in working examples of thisinvention exhibited outstanding properties as compared with the sintersobtained in Comparative Examples as is evident from the fact that theirstrengths were not less than 1100 MPa and their fracture toughnesseswere not less than 11 MPa·m^(1/2).

What is claimed is:
 1. A method for the production of a body of sinteredsilicon nitride, comprising:mixing powdered silicon nitride with asintering additive in a solvent, thereby forming a slurry; mixing intothe slurry, elongated, single crystal β-Si₃ N₄ seed particles, whichhave a larger diameter than the average particle diameter of saidpowdered Si₃ N₄ and having an aspect ratio of at least 2, in an amountof 0.1 to 5% by volume based on the total amount of said powdered Si₃ N₄and said sintering additive, thereby forming a fine dispersion of theseed particles in the slurry; molding the slurry containing seed siliconnitride particles into a green body, which process orients the seedsilicon nitride particles in a specific direction; and heating the greenbody to densify it and to simultaneously induce epitaxial growth of saidsingle crystal β-Si₃ N₄ grains, thereby forming a strong and toughsintered silicon nitride body.
 2. The method according to claim 1,wherein the maximum average particle diameter of said silicon nitridepowder is 0.5 μm.
 3. The method according to claim 1, wherein saidsintering additive is at least one member selected from the groupconsisting of MgO, CaO, Al₂ O₃, Y₂ O₃, Yb₂ O₃, HfO₂, Sc₂ O₃, CeO₂, ZrO₂,SiO₂, Cr₂ O₃, and AlN.
 4. The method according to claim 3, wherein saidsintering additive is a rare earth element oxide.
 5. The methodaccording to claim 1, wherein the maximum aspect ratio of said elongatedsingle crystal β-silicon nitride particles is
 50. 6. The methodaccording to claim 1, wherein the molding step is a sheet molding step.